Mums who cosleep/bed share

How do you do so safely? We are staying with my PIL  and my 12 week old daughter won't sleep in their portacot for more than 20 minutes at a time. She has been sleeping next to me on her swaddle and sleeping bag, but I am getting such a sore shoulder from holding her. If anyone can give tips or photos of positions you use, or ways to ensure their safety that would be great. Do you just have them under the covers with you?